1. Field of the Invention
The disclosed invention resides broadly in the field of waste heat recovery and more specifically in the field of waste heat recovery from the flue pipe of a boiler or furnace. The disclosed apparatus is particularly useful for recovering heat from the flue pipe used for venting the waste products of combustion in hot water boilers and heating systems generally found in domestic use.
The apparatus is intended to be equally compatible with new installations as well as to be retrofitted to an existing system.
2. Prior Art
It has been known for some time that the heat which is lost through the flue pipe or exhaust stack is recoverable and may be used for a number of different purposes such as, providing hot water in a home, providing hot water for use in an industrial establishment, or providing heat in either industrial or home use. However, the majority of prior art systems have required that a separate or independent system be constructed to use the heat recovered from the flue pipe or that a storage tank or the like be used to store water which has been heated in this manner.
The following are prior art examples of attempts to recover and use waste heat found in exhaust gases.
U.S. Pat. 2,035,341 entitled, "WATER HEATER", issued on Mar. 24, 1936 to William A. Radloff. The device disclosed in U.S. Pat. No. 2,035,341is a system add-on which encircles the flue pipe with a plurality of water or fluid containing conductor tubes to capture heat which is radiating from the flue pipe. In an alternative embodiment there is a manifold or fluid containing arrangement which is to be located within the flue pipe. However, in both embodiments the utilization of the recovered heat is through an auxillary storage tank or heating system.
U.S. Pat. No. 2,173,887 entitled, "FLUE GAS WATER HEATER", issued on Sept. 26, 1939 to James I. Shute. U.S. Pat. No. 2,173,887 discloses a waste heat reclamation system for a water heater which comprises a water flue tube gas water heater connected to an open storage tank whereby free circulation of the fluid is had through the natural circulation of the fluid.
U.S. Pat. No. 2,189,749 entitled, "WATER HEATER" issued on Feb. 13, 1949 to Fred T. Windheim and John Windheim. U.S. Pat. No. 2,189,749 provides a domestic water heater and a boiler booster in a single unit. The water supply to the domestic hot water heater unit is first pre-heated in the boiler unit which holds the hot water supply in check until the water temperature reaches a pre-determined level, at that point the water is permitted to enter the auxillary system whereby the temperature of the water is increased utilizing the heat recovered from the flue gas.
U.S. Pat. No. 4,037,567 entitled, "WATER HEATER SYSTEM INCLUDING RECYCLE LOOP", issued on July 26,1977 to Peter L. Torres. U.S. Pat. No. 4,037,567 discloses a hot water heater having a re-circulation loop which draws off a portion of the water being fed to the boiler and circulates it in a recycle loop to extract heat from the waste gases. After heating, that portion of water is mixed with the water supply being fed to the boiler and thereby raise the temperature of the water supply being fed to the boiler unit.
A difficulty with the prior art waste heat recovery systems have been that they have failed to recover all of the available waste heat and to address the problems of water temperature differential during start-up of a boiler or furnace system. During the start-up period there is a tremendous amount of heat available at the flue which may be recovered and utilized to raise the temperature of the water in the recovery unit. However, after extracting the waste heat, difficulty is encountered in returning the heated water from the coil to the main boiler supply, because of temperature differential. This water temperature differential is due to the larger volume contained in the main boiler and the time required to raise it to a comparable temperature range. The result of different temperature levels mixing creates a banging or knocking problem. Additionally, when attempting to recover maximum heat from the flue pipe it is important to maintain the temperature of the flue pipe for natural updraft and to prevent a dewing condition within the flue pipe.